RICE RATOONING - IRRI books - International Rice Research Institute

MORPHOLOGY AND PHYSIOLOGY OF RATOON RICE 37Delayed main crop senescence might be an appropriate selection criterion forgood ratooning ability (8). Delayed senescence is probably coupled with increasedcarbohydrate content of main crop stubble, which could cause better ratoondevelopment. Carbohydrate concentration at harvest and ratooning ability areclosely associated (8, 37).The percentage of ratoon tillers correlated positively (r=0.26*) with carbohydrateconcentration at harvest (8). However, actual number of ratoon tillersappeared to be independent of carbohydrate concentration at harvest. Percentage ofratoon tillers, ratoon weight, and ratoon height correlated positively with stem baseweight. Total nonstructural carbohydrate (TNC) may limit ratoon regrowth (20).Growth regulators were applied to delay main crop senescence, allowing morecarbohydrate to accumulate through photosynthesis (33). Their limited effect onratoon rice indicated that the original hypothesis, that ratooning ability is associatedwith plant carbohydrate content at harvest, might not be correct.In the cultivar BPI76, carbohydrate content correlated positively (r=0.88**)with main crop growth duration (13). However, ratooning ability was associatednegatively with growth duration. Carbohydrate concentration was 48% less in shortgrowth duration plants (117 d) than in 177-d plants, but the short-duration plantshad higher ratooning ability. The 100% stubble regeneration of short-durationplants could not be attributed to reserve carbohydrate content. High regeneration inshort-duration plants might have been due to the high N concentration in thestubble. High N is associated with profuse tillering.N content of the main crop stubble correlated negatively (r=-0.98**) withgrowth duration (13). In Mingolo, carbohydrate content in the stubble correlatedwith the number of ratoon tillers.Carbohydrate content of the main-plant stubble correlated negatively withdegree of shading, which probably explains why plants ratoon poorly if the maincrop is shaded (13). C:N was highest at no shading and lowest at 66% shading.However, C:N in BPI76 increased with longer growth duration; growth durationwas negatively associated with ratooning ability. These data contradict theassumptions about the role of carbohydrates in ratooning ability.These findings suggest that in addition to carbohydrate content of the stubble,other factors may encourage high ratooning. For example, stubble N content maybe as important as carbohydrate content for good ratoon tiller regeneration. Therole of stubble carbohydrate and N content in ratooning ability needs to be clarified.SUMMARYRatooning ability is a potentially important varietal character. But it is acomplicated trait. Improving ratooning ability is difficult because of the complexgenetic architecture and dominating influence of agroclimatic conditions andmanagement practices.No serious, systematic attempt has been made to identify the factors and tounderstand the mechanisms affecting ratooning ability and yield. The factors knownto determine ratooning ability directly or indirectly are summarized in Figure 2.